Field of the Invention
The present invention relates to golf balls. Specifically, the present invention relates to golf balls that include a core and a cover.
Description of the Related Art
Golf players' foremost requirement for golf balls is flight performance. In particular, golf players place importance on flight performance upon a shot with a driver. Flight performance correlates with the resilience performance of a golf ball. When a golf ball having excellent resilience performance is hit, the golf ball flies at a high speed, thereby achieving a large flight distance.
An appropriate trajectory height is required in order to achieve a large flight distance. A trajectory height depends on a spin rate and a launch angle. With a golf ball that achieves a high trajectory by a high spin rate, a flight distance is insufficient. With a golf ball that achieves a high trajectory by a high launch angle, a large flight distance is obtained. Use of a core having an outer-hard/inner-soft structure can achieve a low spin rate and a high launch angle.
Golf balls for which a hardness distribution of a core has been examined in light of achievement of various performance characteristics are disclosed in JP2012-223569 (US2012/0270680), JP2012-223570 (US2012/0270681), JP2012-223571 (US2012/0270679), and JP2012-223572 (US2012/0270678).
JP2012-223571 discloses a golf ball that includes a core having a three-layer structure. In the core, a first layer, a second layer, and a third layer are formed from the central point of the core toward the surface of the core. The hardness gradient of the third layer of the core is greater than the hardness gradient of the second layer. JP2012-223569, JP2012-223570, and JP2012-223572 also disclose similar golf balls. In the core of the golf ball disclosed in JP2012-223569, the hardness of the second layer at a boundary portion between the first layer and the second layer is less than the hardness of the first layer. In the core of the golf ball disclosed in JP2012-223570, the hardness of the third layer at a boundary portion between the second layer and the third layer is less than the hardness of the second layer. JP2012-223572 discloses a core in which the hardness of the second layer at a boundary portion between the first layer and the second layer is less than the hardness of the first layer and the hardness of the third layer at a boundary portion between the second layer and the third layer is less than the hardness of the second layer.
In recently years, golf players' requirements for flight performance have been escalated more than ever. A golf ball with which a large flight distance is obtained upon a shot with a driver without impairing excellent performance such as approach performance, feel at impact, and the like, is longed for. The inventors of the present invention have found that a hardness gradient in a specific region of a core contributes to an increase in a flight distance upon a shot with a driver, and have completed the present invention by optimizing the hardness distribution of the entire core.
An object of the present invention is to provide a golf ball having excellent flight performance.